1. The Field of the Invention
The present invention relates generally to a method for forming a cladding layer in a top conductor, particularly useful with magnetic RAM structures, and a magnetic memory device having a cladding structure about the top conductor.
2. The Background Art
A magnetic memory, such as a magnetic random access memory (MRAM), typically includes an array of magnetic memory cells. Each magnetic memory cell usually includes a sense layer and a reference layer. The sense layer is usually a layer or film of magnetic material that stores magnetization patterns in orientations that may be altered by the application of external magnetic fields. The reference layer is usually a layer of magnetic material in which the magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in a particular direction. The magnetic memory can be described as including a number of bit lines intersected by a number of word lines. At each intersection, a thin film of magnetically coercive material is interposed between the corresponding word line and bit line. The magnetic material at each intersection forms a magnetic memory cell in which a bit of information is stored.
The logic state of a magnetic memory cell typically depends on its resistance to electrical current flow. The resistance of a magnetic memory cell depends on the relative orientations of magnetization in its sense and reference layers. A magnetic memory cell is typically in a low resistance state if the overall orientation of magnetization in its sense layer is parallel to the orientation of magnetization in its reference layer. In contrast, a magnetic memory cell is typically in a high resistance state if the overall orientation of magnetization in its sense layer is anti-parallel to the orientation of magnetization in its reference layer.
It is desirable to reduce the size and increase the packing density of the memory cells achieve a significant density. A number of competing factors influence the packing density that can be achieved for such a memory. A first factor is the size of the memory cells. The size of the memory cells must typically decrease with increased packing density. Reducing the size of the memory cell, however, increases the field required to switch the orientation of the magnetization of the sense layer.
A second factor is the width and thickness of the word and bit lines. The dimensions of the word and bit lines must typically decrease with increased packing density. Reducing the dimensions of the word and bit lines, however, reduces the current that can be accommodated thereby, and thus the magnetic field at the corresponding magnetic bit region.
A third factor is the distance between the word and bit lines, and thus the distance between adjacent memory cells. Typically, the distance between word and bit lines must decrease with increased packing density. However, this increases the possibility that the magnetic field produced by one line may adversely affect the information stored in an adjacent memory cell.
It has been recognized that it would be advantageous to develop a magnetic memory with write conductors that produce enhanced write fields. In addition, it has been recognized that it would be advantageous to develop a magnetic memory with flux closure structures that prevent disruptions to magnetization. In addition, it has been recognized that it would be advantageous to develop a method to fabricate such conductor structures. To this end, it has been recognized that it would be advantageous to clad the top conductors of the magnetic memory.
The invention provides a method for cladding two or three sides of a top conductor in ferromagnetic material for a magnetic RAM structure. The memory device can be provided on a substrate with a bottom conductor also in cladding. The method includes forming a trench above the memory device in an insulating coating layer formed over the memory device and the substrate. The trench includes side walls formed by the coring layer, and a bottom which may be formed by a top surface of the memory device. A first ferromagnetic material is deposited on the coating layer, and in particular, along the side walls of the trench. If desired, any of the first ferromagnetic material deposited in the bottom of the trench can be removed. A conductor material is deposited in the trench, and on the coating layer. If desired, any conductor or ferromagnetic material on the coating layer is removed. Thus, at least the two sides of the conductor are clad in the ferromagnetic material. In addition, a bottom of the conductor also may be clad.
A second ferromagnetic material can be deposited over the conductor material in the trench to form a cladding of the ferromagnetic material around the sides and top of the conductor. The second ferromagnetic material also can be deposited on the first ferromagnetic material along the side walls of the trench to form a continuous cladding. Furthermore, the top surface of the conductor can be recessed prior to depositing the second ferromagnetic material.
The step of removing the first ferromagnetic material from the bottom of the trench can include ion etching of the ferromagnetic material to leave the ferromagnetic material along the side walls of the trench.
The step of removing conductor or ferromagnetic material from the coating layer preferably includes polishing the conductor material, such as with a chem-mechanical polishing process. The step of polishing the conductor material preferably includes creating an indentation or recess in the conductive material in the trench which extends to an elevation below an upper surface of the coating layer.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawing, which together illustrate by way of example, the features of the invention.